According to the Hong Kong Health Department, the DC-CIK procedure involves the "concentration and processing of blood taken from the person, and subsequent infusion of the mixture back into the patient."

It's not clear whether any of the women who received the treatment had been diagnosed with cancer, though the Hong Kong Health Department said in an earlier statement that the 46-year-old had been in "good past health."

The woman, who has only been identified by authorities by her surname "Chan," was admitted to hospital last Thursday suffering fever and diarrhea, hours after receiving the treatment.

She was diagnosed with septic shock, and on Sunday authorities announced her blood samples had grown the bacterium Mycobacterium abscessus.

Dr. Ho Pak-leung, President of the University of Hong Kong's Center for Infection told CNN that, under normal circumstances, M. abscessus did not pose a threat to healthy people.

"It's common, it's widespread. It can be found in dust and soil and water. People often come into contact with a small number of this bacteria and it's of no harm at all unless you have a wound," Dr. Ho said.

However, he said it appeared as though the bacteria had entered the woman's blood.

"For blood infection involving internal organs, it's very difficult. It has to involve multiple drugs given at the same time and there are uncertainties about the effectiveness of the treatment," he told CNN on Monday.

Three other women remain in hospital after having the treatment, health authorities said Wednesday. A 60-year-old is in critical condition, a 56-year-old woman is in serious condition, while another, 59, is stable.

Dr. Ho said the use of the DC-CIK treatment for cosmetic purposes was not something he'd ever heard of before.

"I have serious doubts about the medical and scientific basis of the treatment," he said.

Professor KY Yuen, Chair of Infectious Diseases at the University of Hong Kong's Department of Microbiology, said the procedure was "only indicated for patients with metastatic cancer and often as salvage therapy when there are no other feasible options."

The Hong Kong Health Department and police are investigating the matter but, as of Wednesday morning, police said no arrests had been made. Health authorities have instructed the DR beauty clinic to halt the procedure.

Over the weekend, the founder of DR, Dr. Stephen Chow Heung-wing, told local media that his staff referred clients to an independent doctor who administered the therapy.

He denied suggestions that it was offered as a medical treatment.

The clinic has yet to respond to CNN requests for reaction to Wednesday's death.

"I didn't say that it can cure any diseases. Remember that we are a beauty clinic. We refer clients to doctor(s), and we have had the clients to sign a liability-free form, which states that it is not for medical use," he said.

A statement on the beauty clinic's website says that the treatment "was conducted by an independent doctor at a medical clinic, and the doctor is not hired by our beauty store."

The statement also said it was up to the independent doctor to decide whether the client is suitable for treatment before he or she receives it. It added: "We will assist related government departments to find out the causation of this particular incident."

According to its website, the DR group of clinics was established in 1995 and provides services for more than 1,000 clients each day.